Hi. Thanks for using JA! I will try and help you out.
I am not sure what kind of MPG's you are getting now, but I can tell you that while driving, there are some things you can do that can help fuel economy and get the most power from the engine.
First, try and keep the RPM's as low as possible when driving. By keeping the RPM's low, you will use less fuel and the engine will have more torque. All inline 6 cylinder truck engines are design on a gear fast run slow concept. The Higher the RPM's, the worse fuel economy the engine will get and the less torque output the engine will have.
Another thing to do while driving is to only use as many RPM's as needed to get out of a gear and into the next gear. This is called progressive shifting. Many times drivers will top put on the RPM's and then wait until the RPM's fall to shift gears. This is not efficient. Get through the gears as quick as possible only using as many RPM's as needed to get through the gears.
Please let me know if you need more help or have more questions.
I see. Thanks for the information.
Your 60 series engine should be getting better fuel economy than 5.5. That is not good at all for a 60. When it is driven correctly, you should be getting much better fuel economy.
The engine can be up rated to increase power, however that will not help with fuel economy.
If you are getting 5.5, you most likely have worn injectors. Worn injectors can cause poor engine response and poor fuel economy. I have seen this before.
Please let me know if you have more questions or need more help.